Dalton's Atomic Theory

It was in the early 1800s that John Dalton, an observer of weather and
discoverer of color blindness among other things, came up with his atomic theory. Let's
set the stage for Dalton's work. Less than twenty years earlier, in the 1780's, Lavoisier
ushered in a new chemical era by making careful quantitative measurements
which allowed the compositions of compounds to be determined with accuracy. By 1799 enough
data had been accumulated for Proust to establish the Law of Constant Composition
( also called the Law of Definite Proportions). In 1803 Dalton noted that oxygen and
carbon combined to make two compounds. Of course, each had its own particular weight
ratio of oxygen to carbon (1.33:1 and 2.66:1), but also, for the same amount of carbon,
one had exactly twice as much oxygen as the other. This led him to propose the Law
of Simple Multiple Proportions, which was later verified by the Swedish chemist
Berzelius. In an attempt to explain how and why elements would combine with one
another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated
his atomic theory.

The idea of atoms had been proposed much earlier. The ancient Greek philosophers had
talked about atoms, but Dalton's theory was different in that it had the weight of careful
chemical measurements behind it. It wasn't just a philosophical statement that there are
atoms because there must be atoms. His atomic theory, stated that elements
consisted of tiny particles called atoms. He said that the reason an element is
pure is because all atoms of an element were identical and that in
particular they had the same mass. He also said that the reason elements
differed from one another was that atoms of each element were different from one
another; in particular, they had different masses. He also said
that compounds consisted of atoms of different elements combined together.
Compounds are pure substances (remember they cannot be separated into elements by phase
changes) because the atoms of different elements are bonded to one another somehow,
perhaps by hooks, and are not easily separated from one another. Compounds have
constant composition because they contain a fixed ratio of atoms and each atom
has its own characteristic weight, thus fixing the weight ratio of one element to the
other. In addition he said that chemical reactions involved the rearrangement of
combinations of those atoms.

So that, briefly, is Dalton's theory. With modifications, it has stood up pretty well
to the criteria that we talked about earlier. It did not convince everyone right away
however. Although a number of chemists were quickly convinced of the truth of the theory,
it took about a half century for the opposition to die down, or perhaps I should say die
off.

Let me point out again the difference between a model of atoms and a theory of atoms. A
model focuses on describing what the atoms are like, whereas the theory not only talks
about what the atoms are like but how they interact with one another and so forth. Dalton's
model was that the atoms were tiny, indivisible, indestructible particles
and that each one had a certain mass, size, and chemical behavior that
was determined by what kind of element they were. We will use that model of an atom for
now, but we will modify it considerably in a later lesson.